Amplifying



Patented Oct. 9, 192$ TATES ATENT OFF-ICE.

' RAYMOND A. nnisme, or miLLBUnN, NEW .nmsnv, ASSIGNOB 'ro ,WESTERNELEC- rnrc- COMPANY, rNcoarom'rnn or NEW YORK, N. Y., A. CORPORATION orNEW YORK.

AMPLIFYING.

Application filed December 80, 1822. Serial No. 609,932.

This invention relates to the amplifica-- tionof modulated waves andmore particularly to the amplification of modulated high frequency Wavesfor radio transmission.

It is well knownhow high frequenc waves may be modulated in accordancewit speech-or other signals to roduce a modulated wave from which t eunmodulated carrier component is suppressed or how the unmodulatedcarrier component may be suppressed after modulation. Such a modulatedwavewith the unmodulated component of carrier frequency suppressed isoften referred to as a pure modulated wave. Oftentimes, for one reasonor another, one of the side bands or side frequencies resulting frommodulation is also suppressed, and the remaining wave is then referredto as a single side frequency without unmodulatedcarrier. In the presents ecification, any modulated wave from whic the unmodulated carrierfrequency component has been entirely or largely suppressed will bereferred to as a' zero carrier modulated wave or zero this difficulty issurmounted b carrier wave. This definition is intended to include a wavewherein both side frequencies are present or wherein only one sidefrequenc is present.

A di culty present in amplifier systems in general and especiallyimportant in -amplifier systems for amplifying considerable power, as inradio transmission, is the inefficiency of vacuum tubes when used asam--plifiers. For efficiency a high negative voltage must be impressed uponthe grid and considerable distortion occurs. In a system heretoforedevised and used by applicant, operating the tube at such a point on itsc aracteristlc as to amplify the modulated portion only of anincompletely modulated wave.

In order to use vacuum tube amplifiers of high efficiency for am'lifying zero car-' rier waves, other di culties which arise must beovercome. Llhese difficulties and the method of surmounting them will bediscussed hereinafter.

- One system described herein for exempliffying the inventiori consistsof an arrangement in which zero carrier modulated waves are supplied toa space discharge amplifier of the usual type havin an anode-cathodeoutputcircuit and a gri -cathode input'ei'rcuit. The grid is adjusted toa negativepos tential such that space current just begins to flow whenno waves to be amplified are applied to. the input circuit. A rectifieris connected; across the input circuit in such a manner that theapplication of waves to be am lified to the input circuit causes thegrid to ecome more no ative as the amplitude of the applied waves ecomeslar er. The system operates without appreciable loss of the signalingfrequency components by which the carrier wave is modulated and ishighly efficient.

In another system, current of the modulating wave form is rectified andap lied to the the grid potential thereof to become more negative as theamplitude of the applied Waves becomes larger and vice versa, to 7 causethe grid potential to become less negative as the amplitude of theapplied waves becomes smaller.

It is an object of this invention to amplify zero carrier waves byvacuum tube amphfier systems working at highef'ficiency. In thisspecification, the ex ression high efliciency is used to descri e thecondition of o 'eration in which the ratio of the energy of t e desiredwave form produced in the output circuit to the total energy supplied tothe amplifier system is hi h.

The novel features believe to be characteristic of the invention are setforth with particularity in the appended claims' The lnvention itself,hoWever,, both as to organization and method of operation, together withfurther objects and advantages thereof will best be understood from thefollowing detailed description taken in connection wit the accompanyingdrawings wherein Fig. 1 is a circuit diagram of a system foramplifying'zero carrier waves with high eflicienc Fig. 2 is a diagramillustrating high e ciency amplification of ordinar modulated waves by avacuum tube amplifier system Fig. 3 is a diagram illustrating theinapplicability of the method illustrated in Fig.

2 to zero carrier modulated waves; Fig. 4: 1s a dlagram indlcating themode of operaamplif ing zero carrier modulated waves.

In ig. 2, the curve 0 represents the grid- ,voltage-plate-currentcharacteristic of a vacuum tube amplifier in which grid voltages areplotted along the axis OX and late currents are Eplotted along the axis"Y. The valueof represents the normal negative id voltage to be usedwhen the amplifier notions to amplify an incompletely modulated highfrequency wave represented by the curve w, this wave being modulated inaccordance with a. speech frequenc wave s. When the wave is applied tothe input circuit of the amplifier the id voltage will at times durineach high equency cycle become of suc value that space current will flowin the output circuit. This will result in the production in the outputcircuit of a substantially completely modulated high frequency wave ofslightly distorted wave form. For radio transmissionthe distortionintroduced is a disadvantage, but not such a disadvantage as to overcomethe advantage inherent in this method of amplification.

A consideration of the wave form of the 1 high frequency modulated waveapplied to the grid circuit in accordance with Fig. 2

- as compared with the wave form of a zero carrier modulated waveindicates at once that the method of Fig. 2 is inapplicable for theamplification of zero carrier modulated waves, inasmuch as the wave formof the latter is such that the-envelope of the high frequency wave whichrepresents the wave form of modulating wave is constantly descending toand crossing and recrossing the zero axis. Thus in Fig. 3 the curve arepresents a zero carrier modulated wave consisting of both side bandsand 8 represents the modulating wave. It appears at once that any systemof amplifying in which only the higher peaks of the wave 2' areamplified will result in a loss of important signaling components. Thus,for example, it would not be desirable to amplify only those portions ofJ the wave indicated as lying to the right of the dotted line mn.Furthermore, if all portions of the wave lying to the right of thecentral axis MN are amplified, the system will be inefiicient from. thestandpoint of power consumption.

Consider now the mode of operation indicated in Fi 4. In this case thenormal negative gri voltage E is adjusted to that which space currentbegins to flow as in the case of Fig. 2. This results in goodamplification of those portions of the zero carrier modulated wave ofsmall amplitude. If now the negative rid voltage is caused to becomeconsiderab y more negative during those portions of the speech frequencycycle when the zero carrier wave is of large amplitude, efficiency ofoperation and absence of signal destroying distortion will both result.In order to accomplish this, the negative grid voltage E is caused tovary in accordance with the heavy curve MN as indicated in Fig.

4. The wave form'of the curve-MN sugests at once that it is periodic andthat its undamental frequencyis twice that of the signaling wave a. Itis therefore contemplated that any method of operation or means forperforming. such method whereby the grid voltage an amplifier foramplifying a zero carrier wave is caused to pass through variations atdouble the modulation frequency of the modulating wave is an embodimentof the present invention. It also appears that the principles of theinvention are applicable in amplifying any modulated wave of which theamplitude at times approaches zero, i. e. complete modulation.

Arno g the possible arrangements functioning in accordance with thediagram of Fi 4, is the circuit arrangement indicated.

inFi 1, which will now be described.

11 circuit arrangements comprise an oscillation enerator which maygenerate any desired frequency of carrier wave. By way of illustration,it may be assumed that the frequency produced by the oscillationgenerator 1.is 100,000 cycles per second. The microphone circuit 2typifies any desired source of modulating Waves, oscillations, orimpulses in accordance with which the oscillations produced by thegenerator 1 are to be modulated. The vacuum tubes 3 and their associatedcircuit arrangements constitute a modulator 3 of the well-known carriersuppression type. The function of the modulator is to modulate the wavesfrom the genig. 1, the tube 1 and its associated erator 1 in accordancewith waves from the source 2 and largely suppress the unmoduallel, thezero carr'er waves produced by the modulating system may be separatedfrom any interfering waves y means of a selective circuit arrangement 6,and increased in energy in one or more stages of amplification by meansof vacuum tube amplifiers 7 and 8. The amplifiers 'Z and 8 are of aconventional type well known in the art and need not detaileddescription. The selective circuit 6 preferably consists of a band passfilter of the t pe described in U. S. patent to Campbell 0. 1,227,113,granted Ma 22, 1917. The filter 6 may pass a range 0 freqlilienciesconsisting of the lower side band, t at is, for example, from 98,000 to99,800 cycles per second; or the upper side band, that is, from 100,200to 102,000 cycles per second; or it may pass both side bands, that is,from 98,000 to 102,000 cycles per second.

In the present instance, we will assume that both side bands aretransmitted by the filter 6 and that the unmodulated carrier com onentiswholly suppressed by the modu ator 4 3 and 4. The curve 2 in Fig. 4represents same modulated wave as the curve 2 in Fig.

' 3 but in the case of Fi 4, the-zero axis'of I rectifier.

the wave 2 is curve instead bf being straight as in Fig. 3.

It is in the power amplifying system 4 that large amounts. of power aredissipated, this power being supplied from the space current source 9.The ower consumption in the amplifiers 7 and 8 is too small tonecessitate the application of the invention to their input circuits. Inaccordance with the present invention, the battery 10 should be adjustedto or at about the point where-space current flows in the amplifiertubes 5. A rectifier 11, preferably in the form of an evacuated-tubehaving a cathode .and an anode, is connected across between the rids andthe filaments of the tubes 5 so that incoming waves will be partiallyrectifiedin a manner tending to make the grid more negative. The greaterthe amplitude of the waves, the more negative the grids will become.Hence, when waves of large amplitude are being amplified, the efficiencyis correspondingly high and when waves of small amplitude are beingamplified the tubes work only on the lower part of theircharacteristicand but little energy is dissi ated. See paper Modulationin radio telep ony, lroc. I. R. E. August 1921, Appendix. It ispreferred to include ter 10 in series with a high resistance 12, an toshunt the rectifier 11 around the resistance 12. Another resistance 13is preferably included in circuit with the rectifier to determine thepotential applied to the The resistances 12 and 13 should be varied bothwith respect to each other and the other constants of the system, untila desired operating condition is obtained.

The operation of the system is indicatedin Fig. 4 and has been describedabove. The grid potential of i value at which space current just beginsto flow. Incoming high frequency waves then have some of their currentrectified in the rectifier 11 and cause the grld potential of the tubes5 to fall to a still more negative .value. During those parts of themodulating wave cycle when the high frequency waves are of smallamplitude the negative charge --on the grid caused by the rectifiedcurrent will leak off through the resistance 12. The curve MN (Fig. 4)represents the variation of grid potential. The negative potential ofthe grid thus rises and falls in such a manner that when large waves areamplified, the grid is at such a high negative potential the batv thetubes 5 is set at the as to create an eflici'ent operatin condition.When small waves are ampli ed, only a small amount of power is needed,hence the average efiiciency over .a complete low frequenc cycle remainsfairl high. Whatever istortion is introduce does not result in loss ofessential elements of the voice wave. The high ower waves in the outputcircuit are trans erred to the antenna circuit 14 by means of thecoupling between thecoils l5'and 16..

trated'is one of the arrangements, but by no means the only arrangementwhich may be devised for carrying out this method.

Thus, in the circuit of Fig. 5, a portion of the modulating wave energyis utilized to control the grid potential of the power amplifier.Modulating waves are applied from the microphone .circuit 2 to themodulator 3 which is supplied from source 1 with a carrier wave to bemodulated. The resultant zero carrier modulated waves are applied to theinput circuit'of a power ampli} fier system 4. Some of the speechfrequency energy from the circuit 2 1s amplified by the amplifier 17 andapplied to a double rectifier system 18 consistingof two hot filamentevacuated tubes 19 and 20. The tubes 19 and 20 rectify the alternatehalf waves of the alternating current of speech frequencyv the amplifier17. and thus cause supplied by the power tubes of the system the gridsof ,4 to vary in potential in accordance with the curve MN. of Fig. 4.The resistance 21 serves asan impedance element across which the\e'lectiomotive force resulting from the rectifigg current is applied tothe grid circuit of e tubes 4.

The novel features which are believed to be inherent in the inventionare defined in the appended claims.

What is claimed is:

1. The method of repeating at high efiiciency, modulated wave by meansof a space dischar e repeater havin a control element whic comprisessupp ying to the control element a negative voltage of such a value asto just allow the repetition of waves of small amplitude andautomatically increasing this negative voltage in a ratio proportionalto the magnitude of the waves when waves of large amplitude are to berepeated. I

2. The method of repeating a zero carrier modulated wave comprising acarrier wave as regards power consumption, a

control element which comprlses var inggrrid periodithe polarizingpotential of the equency of cally at a frequency twice the the lowerfrequency wave. 7 a

3. The method of repeating a wave of a given frequency having amplitudevariations in accordance with a modulating wave of a given frequency bymeans of a space discharge device having a grid or control element whichcomprises varying the potential of the grid periodically at a frequencytwice the frequency of the modulating wave. 4. The method of repeating ahigh frequency wave of-variable amplitude by means of a space dischargedevice which comprises rectifying a portion of the high frequency waveenerg by means of a device other than the space ischarge device andapplying to the space discharge device a potential difference resultingfrom the rectification to control the repetition of thehigh frequencywaves by the space discharge device.

5. The method of high frequency wave amplification by a discharge deviceamplifier containing a control element which comprises applying aportion of the high frequency energy which is to be repeated to arectifier and applying to the control element of the amplifier aresultant negative voltage which "varies. periodically at a frequencytwice that of the high frequency wave.

6. A vacuum tube repeaterhaving input electrodes, a rectifier connectedbetween said electrodes, a resistance in series with said mew.

rectifier, and ahi h resistance connected between said electro es inparallel to said rectifier which supplies rectifiedcurrent theretoforapplying a varying potential between said electrodes.

7 In combination a vacuuin tube repeater for repeating only impulses ofa zero carrier wave of one polarity, a rectfiier in shunt thereto forvarying-the grid potential of said repeater at a frequency double thefrequency of the modulating wave, and a ance in series with saidrectifier. 1 I 8. The method of amplifying a wave having components offixed and variable freresistquencies by means of a space dischargedevice having an anode-cathode and control electrode or grid whichcomprises varying the grid polarizing potential as a function of theamplitude of one of the wave coinponents. f

9. A vacuum tube repeater having grid, filament and plate electrodes, aninput circuit including said grid and filament electrodes, means forimpressing-waves to be repeated on said grid and filament, a high IRAYMOND A. HEISING.

